Spatial light modulators or SLMs include an array of one or more devices that can control or modulate an incident beam of light in a spatial pattern that corresponds to an electrical input to the devices. The incident light beam, typically generated by a laser, can be modulated in intensity, phase, polarization or direction. Some modulation can be accomplished through the use of Micro-Electromechanical System devices (MEMs) that use electrical signals to move micromechanical structures to modulate light incident thereon. Spatial light modulators are increasingly being developed for use in various applications, including display systems, optical information processing and data storage, printing, and maskless lithography.
FIG. 1 shows a linear, one-dimensional (1D) array of ribbon-type diffractors for use in a SLM. Generally, the linear array consists of a number of active (movable) ribbons are interlaced between static bias ribbons. By displacing the active ribbons, relative to the static ribbons, a square-well diffraction grating is formed along the long axis of the array. In the embodiment shown, several ribbon pairs are ganged under action of a single channel driver to form a single MEMS pixel. By assembling a large number of MEMS pixels and drivers, a continuous, programmable diffraction grating results, such as is particularly useful in printing and lithography applications.
One shortcoming of existing ribbon-type diffractors is that when a potential difference is applied between the active ribbons and substrate the active ribbons are deflected into a parabolic profile as shown in FIG. 1B. As a result the square-well diffraction grating is established only in a narrow region near the center-line of the array that is truly displaced by a quarter wavelength (λ/4). Regions outside this optical “sweet-spot” are neither parallel to the device surface nor displaced by λ/4 and cannot provide high contrast and high efficiency modulation. For this reason, illumination onto the standard ribbon-type diffractor array must be carefully shaped into a line focus. A rule of thumb is that the width of the illumination should be roughly on the order of 1/10th to ⅓rd the ribbon length, depending on the application and contrast ratio demands.
The need to concentrate the illumination along a narrow line-width in the middle of the array leads to a number of problems. First, line-illumination concentrates laser power in a thin, high power density line, creating a thermal knife-edge having enormous thermal gradients. Moreover, as power density is pushed higher in applications such as in Computer Thermal Printing (CTP) and maskless lithography these thermal gradients can increase to the point where the ribbons begin to fail. Typically, the failure mode is the “Soret effect” in which atoms of a reflective metal, such as aluminum, covering the ribbons physically migrate along from a hotter to a cooler region of the ribbon. This migration of metal atoms can reduce the reflection and hence the efficiency of the SLM, and ultimately shortens useful device life.
Accordingly, there is a need for a new SLM and method of operating the same to provide increased operating lifetime of the SLM.